This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A111491 #6 Dec 11 2012 17:00:54
%S A111491 1,2,5,36,539,16710,1052729,133696584,34092628919,17421333377610,
%T A111491 17822024045295029,36481683220718924364,149392492788843995270579,
%U A111491 1223673908433421165261312590,20047449641864738950476084161969,656894782414981901190249849735238224
%N A111491 a(0) = 1; for n>0, a(n) = (2^n-1)*a(n-1)-(-1)^n.
%D A111491 W. T. Trotter, Combinatorics and Partially Ordered Sets, Johns Hopkins, 1992; see p. 195.
%p A111491 s:=proc(n) option remember; if n=0 then 1 else (2^n-1)*s(n-1)-(-1)^n; fi; end;
%t A111491 RecurrenceTable[{a[0]==1,a[n]==(2^n-1)a[n-1]-(-1)^n},a,{n,20}] (* _Harvey P. Dale_, Dec 11 2012 *)
%Y A111491 Cf. A111968.
%K A111491 nonn
%O A111491 0,2
%A A111491 _N. J. A. Sloane_, Nov 28 2005, typo corrected Nov 21 2008